The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems are used to provide fresh water from brackish or sea water. A membrane is used that restricts the flow of dissolved solids therethrough.
Referring now to FIG. 1, a reverse osmosis system 10 is illustrated having a membrane array 12 that generates a permeate stream 14 and a brine stream 16 from a feed stream 18. The feed stream 18 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to the required pressure flow which enters the membrane array 12.
The permeate stream 14 is purified fluid flow at a low pressure. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. The membrane array 12 requires an exact flow rate for optimal operation. A brine throttle valve 24 may be used to regulate the flow through the membrane array 12. Changes take place due to water temperature, salinity, as well as membrane characteristics, such as fowling. The membrane array 12 may also be operated at off-design conditions on an emergency basis. The feed pumping system is required to meet variable flow and pressure requirements.
In general, a higher feed pressure increases permeate production and, conversely, a reduced feed pressure reduces permeate production. The membrane array 12 is required to maintain a specific recovery which is the ratio of the permeate flow to feed flow. The feed flow or brine flow likewise requires regulation.
A pretreatment system 21 may also be provided to pretreat the fluid into the membrane array 12. The pretreatment system 21 may be used to remove solid materials such as sand, grit and suspended materials. Each of the embodiments below including those in the disclosure may include a pretreatment system 21.
Referring now to FIG. 2, a system similar to that in FIG. 1 is illustrated with the addition of a feed throttle valve 30. Medium and large reverse osmosis plants typically include centrifugal-type pumps 20. The pumps have a relatively low cost and good efficiency, but they may generate a fixed pressure differential at a given flow rate and speed of rotation. To change the pressure/flow characteristic, the rate of pump rotation must be changed. One way prior systems were designed was to size the feed pump 20 to generate the highest possible membrane pressure and then use the throttle valve 30 to reduce the excess pressure to meet the membrane pressure requirement. Such a system has a low capital cost advantage but sacrifices energy efficiency since the feed pump generates more pressure and uses more power than is required for a typical operation.
Referring now to FIG. 3, another system for solving the pressure/flow characteristics is to add a variable frequency drive 36 to operate the motor 12 which, in turn, controls the operation of the feed pump 20. Thus, the feed pump 20 is operated at variable speed to match the membrane pressure requirement. The variable frequency drives 36 are expensive with large capacities and consume about three percent of the power that would otherwise have gone to the pump motor.
Referring now to FIG. 4, a system similar to that illustrated in FIG. 1 is illustrated using the same reference numerals. In this embodiment, a hydraulic pressure booster 40 having a pump portion 42 and a turbine portion 44 is used to recover energy from the brine stream 16. The pump portion 42 and the turbine portion 44 are coupled together with a common shaft 46. High pressure from the brine stream passes through the turbine portion 44 which causes the shaft 46 to rotate and drive the pump portion 42. The pump portion 42 raises the feed pressure in the feed stream 18. This increases the energy efficiency of the system. The booster 40 generates a portion of the feed pressure requirement for the membrane array 12 and, thus, the feed pump 20 and motor 22 may be reduced in size since a reduced amount of pressure is required by them.
Referring now to FIG. 5, a basic low-cost scheme for a large reverse osmosis plant 50 is illustrated using reference numerals similar to those of FIG. 1. In this embodiment, three reverse osmosis stages having three membranes 12a, 12b, and 12c are used together with three throttle valves 30a, 30b, and 30c. Three brine throttle valves 24a, 24b, and 24c are coupled to the brine output 16a, 16b, and 16c. The feed stream 18 is coupled to a feed manifold 52 which, in turn, is coupled to each of the feed throttle valves 30a-30c. Each throttle valve is used to provide feed fluid to each of the respective membrane 12a-12c. The brine stream 16a-16c passes through the brine throttle valves 24a-24c and into a brine manifold 54. The permeate streams are coupled to a permeate manifold 56.
In a large reverse osmosis plant 50, the objective is to use a feed pump with the largest available capacity to achieve the highest possible efficiency at the lowest capital cost per unit of capacity. The optimal capacity of a membrane array 12 is usually smaller than the pumps. Therefore, a single-feed pump 20 may be used to multiple supply membrane arrays 12. Such a configuration is called centralized feed pumping. Because each of the membranes has a variable pressure requirement, individual control using the throttle valves 30a-30c and 24a-24c may be used. However, using throttle valves wastes energy. Also, the individual membranes themselves may have their own pressure requirements due to the following level of the membranes which may vary over the membrane array.
Referring now to FIG. 6, a similar configuration to that of FIG. 5 is illustrated with the addition of a variable frequency drive used to drive the motor 22 and thus the pump 20. The variable frequency drive 60 is used to develop enough pressure at the pump 20 to satisfy the pressure requirements of the membrane arrays with the highest pressure requirement. The centralized pumping is partially offset by the difficulty of customizing the fixed discharge pressure of the feed pump to the variable pressure requirements of the multiple membrane arrays. Both of the configurations in FIGS. 5 and 6 require individual throttling and, thus, the energy efficiency is limited.
Referring now to FIG. 7, an embodiment similar to that illustrated in FIG. 4 may include an auxiliary brine nozzle 70 that is controlled by a brine valve 72. Normal operating conditions of a reverse osmosis plan may require making variations in the brine flow and pressure to keep the membrane array operating at optimal conditions. The brine valve 72 allows the brine flow to be increased to allow additional high pressure brine to pass into the turbine portion 44. If less brine flow is required, the auxiliary brine valve 72 may be closed. The auxiliary brine valve 72 may be manually closed or closed by a valve actuator.
Referring now to FIG. 8A, another prior art embodiment is illustrated that includes a feed manifold 80 that receives low pressure feed fluid. The feed fluid may be provided from a pretreatment system 21 as illustrated in FIG. 1. In this embodiment, a plurality of feed pumps 20 is illustrated. The components set forth may be provided in several redundant systems. The components may be referred to without their alphabetical designations. In particular, three pumps 20a-20c with corresponding motors 22a-22c are provided. The pumps 20 provide fluid at a high pressure to a high-pressure feed manifold 82. For servicing purposes, the pumps 20a-20c may be isolated and taken off line through the use of isolation valves 84 and 86. The isolation valve 84 may be positioned between the low pressure feed manifold 80 and the pump 20. The isolation valve 86 may be positioned between the pump and the high pressure manifold 82.
The pumps 20, the motors 22, and the isolation valves 84 and 86 may be referred to as the high pressure pump portion 90.
An isolation valve 92 may be positioned between the high pressure manifold 82 and the membrane 12. Each of the membrane arrays 12a-12h may include a corresponding input isolation valve 92a-h. A permeate isolation valve 94 may be positioned at the permeate outlet of the membrane array 12. A throttle valve 93 may also be disposed at each membrane downstream of isolation valve 92 to permit regulation of pressure for each membrane array 12. The throttle valves are labeled 93a-93h. An isolation valve 96 may be located at the high pressure brine output of the membrane array 12. Each of the respective membrane arrays may include a permeate isolation valve 94 and a brine isolation valve 96. The membrane arrays 12 and the isolation valves 92-96 may be referred to as the membrane array section 100 of the system.
The permeate outputs of the membranes 12 may all be in fluid communication with a low pressure permeate manifold 102 through the valves 94. The high pressure brine output of the membranes 12 may be in fluid communication with a high pressure brine manifold 104.
A plurality of flow work exchangers (FWE) 110 may be in fluid communication with the high pressure brine manifold 104. The flow work exchanger 110 will be further described below in FIG. 8B.
One output of the flow work exchanger 110 provides a lowered pressure from the brine output to a drain 112.
The flow work exchanger 110 also has an input in fluid communication with the low pressure feed manifold 80. Each fluid work exchanger 110 pressurizes the fluid received from the feed manifold 80 and provides a higher pressure to the high pressure feed manifold 82. The flow work exchanger 110 thus draws feed fluid from the low pressure feed manifold 80 and increases the pressure therein which is discharged into the higher pressure feed manifold 82. Thus, the combination of the output of the pump portion 90 and the output of the energy recovery portion 120 combine to provide the feed flow for the membrane portion 100.
The flow work exchanger 110 thus has two fluid input and two fluid outputs. The fluid input to the flow work exchanger 110 from the brine manifold 104 may include an isolation valve 122. The fluid input to the flow work exchanger 110 from the low pressure feed manifold 80 may include an isolation valve 124. The brine output of the flow work exchanger 110 may include an isolation valve 126. The high pressure output of the flow work exchanger 110 may include an isolation valve 128.
The isolation valves 122, 124, 126 and 128 allow the flow work exchanger 110 to be removed from service without interrupting the operation of the system. In the configuration of FIG. 8, the flow work exchanger 110 can only deliver a feed flow rate that is about equal to the brine flow rate. There is no possibility to increase or decrease the feed flow relative to the brine flow.
Referring now to FIG. 8B, the flow work exchanger 110 is illustrated in further detail. The flow work exchanger 110 may include electrical control equipment 130, booster pumps 133 and other equipment 137, 138. The other equipment may include pistons, valves and pressure vessels. The various components within the flow work exchanger 110 may be connected in various ways depending on the type of components used.
Improving the efficiency of reverse osmosis systems to reduce energy consumption is a desirable goal.